Well, welcome to UU.
This thread
here will help introduce you to some of the biggest recent changes/arguments/problems/etc.
I would become familiar with all of the Pokemon listed as UU in the Strategydex, however, some of the really big ones include:
Venusaur, Heracross, Milotic, Scyther, and Arcanine.
Other notable ones are:
Registeel, Uxie, Mismagius, Alakazam, and Moltres.
Basically all the rage is simply its OU, but with different and cooler Pokemon with a broader range of usable Pokemon, including various NU Pokemon. Also Sandstorm is seen more as a gimmick than viable strategy in UU. The Stealth Rock and Rapid Spin users are different. It's just different.
I would also read some of the threads in progress in the UU Sub-forum in C&C to become familiar with some of the prominent trends and Pokemon.